Incrementing/Decrementing Operators
> <?php
> your_function() or return "whatever";
> ?>
doesn't work because return is not an expression, it's a statement. if return was a function it'd work fine. :/
Logical Operators
| Example | Name | Result |
|---|---|---|
| $a and $b | And | TRUE if both $a and $b are TRUE. |
| $a or $b | Or | TRUE if either $a or $b is TRUE. |
| $a xor $b | Xor | TRUE if either $a or $b is TRUE, but not both. |
| ! $a | Not | TRUE if $a is not TRUE. |
| $a && $b | And | TRUE if both $a and $b are TRUE. |
| $a || $b | Or | TRUE if either $a or $b is TRUE. |
The reason for the two different variations of "and" and "or" operators is that they operate at different precedences. (See Operator Precedence.)
Example #1 Logical operators illustrated
<?php
// --------------------
// foo() will never get called as those operators are short-circuit
$a = (false && foo());
$b = (true || foo());
$c = (false and foo());
$d = (true or foo());
// --------------------
// "||" has a greater precedence than "or"
// The result of the expression (false || true) is assigned to $e
// Acts like: ($e = (false || true))
$e = false || true;
// The constant false is assigned to $f and then true is ignored
// Acts like: (($f = false) or true)
$f = false or true;
var_dump($e, $f);
// --------------------
// "&&" has a greater precedence than "and"
// The result of the expression (true && false) is assigned to $g
// Acts like: ($g = (true && false))
$g = true && false;
// The constant true is assigned to $h and then false is ignored
// Acts like: (($h = true) and false)
$h = true and false;
var_dump($g, $h);
?>
The above example will output something similar to:
bool(true) bool(false) bool(false) bool(true)
add a note
User Contributed Notes
Logical Operators - [9 notes]
Andrew ¶
5 years ago
pepesantillan at gmail dot com ¶
5 years ago
worth reading for people learning about php and programming: (adding extras <?php ?> to get highlighted code)
about the following example in this page manual:
Example#1 Logical operators illustrated
...
<?php
// "||" has a greater precedence than "or"
$e = false || true; // $e will be assigned to (false || true) which is true
$f = false or true; // $f will be assigned to false
var_dump($e, $f);
// "&&" has a greater precedence than "and"
$g = true && false; // $g will be assigned to (true && false) which is false
$h = true and false; // $h will be assigned to true
var_dump($g, $h);
?>
_______________________________________________end of my quote...
If necessary, I wanted to give further explanation on this and say that when we write:
$f = false or true; // $f will be assigned to false
the explanation:
"||" has a greater precedence than "or"
its true. But a more acurate one would be
"||" has greater precedence than "or" and than "=", whereas "or" doesnt have greater precedence than "=", so
<?php
$f = false or true;
//is like writting
($f = false ) or true;
//and
$e = false || true;
is the same as
$e = (false || true);
?>
same goes for "&&" and "AND".
If you find it hard to remember operators precedence you can always use parenthesys - "(" and ")". And even if you get to learn it remember that being a good programmer is not showing you can do code with fewer words. The point of being a good programmer is writting code that is easy to understand (comment your code when necessary!), easy to maintain and with high efficiency, among other things.
Lawrence ¶
5 years ago
Note that PHP's boolean operators *always* return a boolean value... as opposed to other languages that return the value of the last evaluated expression.
For example:
$a = 0 || 'avacado';
print "A: $a\n";
will print:
A: 1
in PHP -- as opposed to printing "A: avacado" as it would in a language like Perl or JavaScript.
This means you can't use the '||' operator to set a default value:
$a = $fruit || 'apple';
instead, you have to use the '?:' operator:
$a = ($fruit ? $fruit : 'apple');
phpnet at zc dot webhop dot net ¶
4 months ago
This works similar to javascripts short-curcuit assignments and setting defaults. (e.g. var a = getParm() || 'a default';)
<?php
($a = $_GET['var']) || ($a = 'a default');
?>
$a gets assigned $_GET['var'] if there's anything in it or it will fallback to 'a default'
Parentheses are required, otherwise you'll end up with $a being a boolean.
dartello at gmail dot com ¶
4 months ago
Unlike in C/C++ the invertor (Not) in PHP assumes a string:
<?php
$a = 1;
$t = !$a;
var_dump($t);
?>
The above example will output:
string(0) => ""
To approach the C/C++ handling, this can be solved as follows:
<?php
$a = 1;
(int) $t = !$a;
var_dump($t);
(bool) $u = !a;
var_dump($u);
?>
The above example will output:
int(0)
bool(false)
peter dot kutak at NOSPAM dot gmail dot com ¶
5 years ago
$test = true and false; ---> $test === true
$test = (true and false); ---> $test === false
$test = true && false; ---> $test === false
NOTE: this is due to the first line actually being
($test = true) and false;
due to "&&" having a higher precedence than "=" while "and" has a lower one
momrom at freenet dot de ¶
4 years ago
Evaluation of logical expressions is stopped as soon as the result is known.
If you don't want this, you can replace the and-operator by min() and the or-operator by max().
<?php
function a($x) { echo 'Expression '; return $x; }
function b($x) { echo 'is '; return $x; }
function c($x) { echo $x ? 'true.' : 'false.' ;}
c( a( false ) and b( true ) ); // Output: Expression false.
c( min( a( false ), b( true ) ) ); // Output: Expression is false.
c( a( true ) or b( true ) ); // Output: Expression true.
c( max( a( true ), b( true ) ) ); // Output: Expression is true.
?>
This way, values aren't automaticaly converted to boolean like it would be done when using and or or. Therefore, if you aren't sure the values are already boolean, you have to convert them 'by hand':
<?php
c( min( (bool) a( false ), (bool) b( true ) ) );
?>
paranoiq at centrum dot cz ¶
5 years ago
and, or and xor can be used as conditional constructs:
<?php
// do_that() is executed only if do_this() returns false
if($something) do_this() or do_that();
// $b is assigned to $b, do_that() is executed if $b is false
if($something) $a = $b or do_that();
// do_that() is executed only if do_this() returns true
if($something) do_this() and do_that();
// $b is assigned to $b, do_that() is executed if $b is true
if($something) $a = $b and do_that();
// both do_that() and do_this() are executed..
if($something) do_this() xor do_that();
// .. so the behaviour is same as:
if($something) {
do_this();
do_that();
}
?>
for understanding what happens if $b is NULL or do_this() returns NULL, read the avbentem's comment on NULL type. generaly speaking, NULL is threated like false in most cases.
f dot sylvestris at gmail dot com ¶
16 days ago
The lesson I take from this manpage is this: use more parentheses than you think you need. Seriously, before I read this, I thought assignment had the lowest possible precedence. Of course, I always parethesize defensively so that I don't have to bother memorizing operator precedences, but now I'm especially glad I developed that habit.